The invention relates generally to robotics and deals more particularly with a control for an electric motor which drives a robotic appendage such as an end effector toward and into engagement with an object, which control regulates the speed at which the appendage approaches the object and the force with which the appendage impacts and/or holds it.
One type of robotic appendage with which this invention is concerned is shown in German Pat. No. 28 51 961 to Stamcek et al and takes the form of a robotic hand used for gripping an object. The robotic hand comprises a body portion, two fingers pivotally mounted to the body portion as a pincer, and a reversible motor which drives the fingers toward or away from each other. Often the robotic hand is carried by a robotic arm which in turn may be supported by other robotic arms or by a robotic trunk, the arms and trunk provide means for moving the end effector in most any direction. The robot may be used to grasp and transport the objects of grasp a tool with which to work on them, for example, a nozzle of a hose leading from a source of spray paint.
To provide a system in which the aforedescribed robot functions rapidly, it is desirable that the robotic fingers converge upon and grip an object in a relatively short period of time. To this end, the motor may be driven at a high speed by a high voltage. However, a high voltage will cause the fingers to contact the object with a high force and many types of objects are fragile and may be damaged by the fingers under such conditions. For example, if the object is made of sheet material or thin, hard plastic or is a soft food item, it is possible for the fingers to mar, dent or even crush it. In addition to the harm done to the object, if the motor is powered by a high voltage and is stalled, the "back EMF" diminishes substantially and current surges through the motor, which current rapidly heats and may damage the motor.
In U.S. Pat. No. 3,477,007 to Ducommun et al, there is disclosed a control for a motor which drives a tool such as a screwdriver. When a screw engaged by the screwdriver is driven to its furthest extreme, the motor stalls and as a result, the back EMF of the rotor diminishes rapidly. Hence, the current through the motor increases rapidly, and the control senses the current increase. When the current exceeds a threshold level, the control triggers a circuit breaker which shuts off the current altogether to prevent overheating and to prevent further application of torque to the screwhead. Note that the screwdriver engages the screw head with a relatively large torque for a short time after the screw motion ceases.
Accordingly, a general aim of the invention is to provide a simple control for an electrically powered robotic appendage which control causes the appendage to rapidly approach an object but contact and/or hold it with a relatively moderate force to prevent damage to the object and to the motor.
A more specific aim of the invention is to provide a control of the foregoing type in which the robotic appendage approaches the object rapidly but slows down before impact.
Another specific aim of the invention is to provide a control of the foregoing types for a motor which drives pincer fingers of a robotic hand.
Still another specific aim of the invention is to provide a simple means for controlling the force by which a robotic appendage impacts an object and grips it if the robotic appendage takes the form of two or more pincer fingers.